Hot corrosion is a pervasive phenomenon in gas turbine engine components. Corrosive salts are typically sulfates, especially sodium and potassium sulfates which are constituents of ingested sea salt and salty dust. With regard to the gas turbine engine especially, inhaled or ingested salt normally does not pose a problem at temperatures in excess of about 1700° F. (927° C.), where the sulfate salts usually vaporize faster than they are deposited and as a result they are exhausted with little or no engine damage. But at intermediate temperatures, in the range from about 1000° F. (538° C.) to about 1700° F. (927° C.), the salts may be present as a molten deposit that is extremely corrosive.
Modern aircraft engine components may be fabricated of nickel or cobalt based “superalloys” that may exhibit relatively high corrosion resistance. However, even these superalloy components are subject to thinning as the salt deposit corrodes away the superalloy. Unless the component is replaced through routine inspection and maintenance procedures, it can become damaged to the extent that its function is adversely affected. Accordingly, the potential damage that may result from hot corrosion imposes routine inspection and maintenance schedules in the aerospace industry, even on parts made from superalloys.
Coatings have been used in an effort to counter act or limit the effect of corrosion on aerospace components. For example, platinum-aluminum (PtAl) or cobalt-chromium-aluminum-yttrium (CoCrAlY) and cobalt-nickel-chromium-aluminum-yttrium (CoNiCrAlY) coatings reduce the rate of corrosion to more acceptable levels. However, CoNiCrAlY coatings are deposited with processes that are incapable of coating the surfaces of internal air-cooling passages within a component. For example, the airfoil coating application processes are “line of sight” processes. These include plasma spraying and electron beam physical vapor deposition (EB-PVD). These line of sight processes have limitations with respect to capability to coat complex internal passage ways in a superalloy airfoil, for example. Internal surfaces may be coated with a thin NiAl layer using a gas phase or chemical vapor deposition aluminizing process, but NiAl is not as protective as the MCrAlY coatings. There has long been an identified need for an improved corrosion resistance within internal passageways.
Air acting as a coolant medium flows through these passageways to maintain the component's structural superalloy substrate in an acceptable temperature range that will reduce the likelihood of temperature-induced harm to the component. Salt or salty dust in the incoming coolant air may form deposits on the internal passage ways. Hot corrosion from these deposits may corrode the internal passage ways. This reduces performance of a component, such as an airfoil for example, and may necessitate the component's premature removal from service.
Accordingly, it is desirable to provide a corrosion barrier coating that is compatible with and adheres well to superalloys. The corrosion barrier coating may also increase the useful life of aluminide or CoNiCrAlY coated superalloy surfaces. The corrosion barrier should be stable at relatively high operating temperatures, such as those encountered in aircraft engines, for example. In addition, it is desirable that the coating be applied in a process that permits the coating of complex geometry surfaces such as internal passage ways of a turbine blade or vane. The coating should also desirably be uniform and thin in some instances to avoid interference with fluid flow in narrow channels of coated objects, for example. Furthermore, other desirable features and characteristics of the corrosion protection coatings will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.